smoke detection Flashcards
Benefit of detecting smoke
it’s an early indication of fire
Where are smoke detectors placed
Cargo holds
toilets
Avionics bay
crew rest area
TRaingle - fire causing agents
- source of ignition
- fuel
- oxygen
Take any of the three away, and fire will go out
Categorization
Level of smoke and fire detection depends on ‘accessibility of the compartments
- How easy the are to access and fix the problem
2 Types of ‘smoke’ detection systems
Optical
Ionizing
Optical detection
Detector is comprised of a light source and sensor arranged in this manner:
In normal conditions
* The sensor can’t detect light source
In smoke condition (particles are present)
* light reflects off the particles and is detected by the sensor
* ** Two or more detectors both sensing** the smoke simultaneously will trigger an alarm
* a labyrinth optical detector - light reflects towards the photsensitive cell and the labyrinth has a shield
How to test if smoke detector is working
In cargo smoke panel, you can test by doing this:
- once you press the test button
- test lamp shines at the sensor
- this triggers the signal and illuminates the caption
Ionizing
A system with ionizing chambers
contains radio active material that emit **alpha particles - they ionize the air **in the chambers.
In normal conditions:
* a potential difference in ionized air allows a small electrical current to flow between the plates of the dector
In smoke conditions
* smoke particles enter the chamber.
* They get absorbed by alpha particles
* ionization level decreases, consequently the current flow decreaes
* Once current flow drops to a certain leve, a warning is triggered.
Fire detection and extinguishing system in
2 key areas
cargo compartment
engine bay
main wheel wells
Auxillary power unit
3 vulnerable zones in an engine bay
IN A 3 ZONE SYSTEM
Requires both fire detection and extinguisher
Zone 1 - Most sensitive to fire
Area of hydraulic pumps, fuel pumps, fuel control unit
Zone 2
compressor & fan area
uncontained blade area can cause fire
Generates more heat than zone 1
Zone 3
combustion and turbine sections
Each zone is spearated by fire proof Bulkhead/FIREWALLS - stainless steel or titanium
IN A 2 ZONE SYSTEM
Zone 1 & 2 = Cool zone
Zone 3 = Hot zone
Auxillary Power Unit
Also an engine
- APU bay - located at the back of the aircraft
- Has it’s own fire detection and protection system
- Automatic
- APU shuts down in case of fire
- Zone 1
Engine Fire detection systems
Continuous Loop
Gas filled detector
Bimetallic Overheat sensors
Continous Loop Engine Fire Detection System (FIRE WIRE)
Operates on the principle of:
* Wire to detect resistance and capacitance when heated
* Therefore there is a
Composed of:
1. Controller monitor
2. Detection loops (stainless steel, salt,nickle core)
* negative coefficient of resistance loop
* positiove coefficient of capacitance loop
Location:
* engine nacelles
* loops are wound around engine components on the engine casing
**Each engine is equipped with: **
* A pair of fire detection loops
* A pair of overheat detection loops
Set up:
2 loops are used: Loop A & Loop B
independent of each other
have their own detection controller
How it works - in resistance system
* current flows in a continuous loop
* If temp rises, the salt resistance reduces
* flow of current between electrode and out tube increases
* At a predetermined temperature, sufficient current will flow and warning system will operate
* If temp drops to a certail value, the system will reset and alarm stops
* This works when there is an isolated temperature increase due to hot gas leak or fire
How it works - in capacitance system
* current flows in a continuous loop
* If temp rises, the capacitance increase
* feedback current increases
* when it increases at a predetermined level, warning system will operate
* If temp drops to a preset value, the system will reset and alarm stops
* This works when there is an isolated temperature increase due to hot gas leak or fire
Detecting faults:
* short circuit between the case and inner elctrode may give a false fire warning
* When a disrepency cannot detect heat
* Ex: If there is a rapid decrease in resistance and corresponding increase in capacitance
Fault Alert
* Amber FAULT caption is illuminated for the relevant loop A or Loop B push button
* Amber Loop light illuminates
What to do after seeing the alert
* Selecting the faulty loop off restores the fire detection capability.
* You must isolate the defective loop because a malfunctioning loop that isn’t turned off, will prevent proper fire alerting
Continous Loop Engine Fire Detection FAULT
Scenario 1:
(low resitance) and (low capacity)
(fire) and (no fire) = fault
Scenarion 2:
Sensing element is crushed, inner core doesn’t touch the other casing
Continous Loop Engine Fire Detection FAULT/Alarm/no effect
Scenario 1: FAULT
(low resitance) and (low capacity)
(fire) and (no fire)
Scenario 2: No Reaction
Sensing element is crushed, inner core doesn’t touch the other casing
Scenarion 3: fire alarm is triggered
Inner core contacts the outer casing
Scenario 4: fire alarm is triggered
Sensing element is crushed,casing is touched
